This invention relates to an improvement in a brake system in which a manual control input such as a pedal stroke is detected and braking is electronically controlled so as to generate an effect such as vehicle deceleration corresponding thereto.
The basic construction of such known types of brake systems is typically illustrated in FIG. 4. The operation of a manual control input means, such as a brake pedal 31, is detected by a pedal stroke sensor or pedal force sensor 32, and the information is fed into an electronic control unit 33. The information from a wheel speed sensor 34 and a gravitation type acceleration/deceleration sensor 35 and a brake pressure sensor 36 is also fed into the electronic control unit. The electronic control unit determines a target deceleration corresponding to the control input based on a predetermined relationship, compares this target with the actual deceleration obtained from the wheel speed sensor, and a gravitation type acceleration/deceleration sensor, if provided, and gives a signal to a hydraulic modulator 37 to increase or decrease the brake fluid pressure. The hydraulic modulator controls the extent of the introduction of power fluid from an auxiliary power source 38 by operating an inlet valve and an exhaust vale 39 contained in the modulator, so as to increase or decrease the pressure prevailing in a wheel brake 40 according to the signal given by the electronic control unit to thus obtain the target deceleration.
This type of brake system has excellent controllability and reliability because the braking effect corresponding to the control input can always be obtained irrespective of the payload on the vehicle or changes in the frictional coefficient of the friction material of the brake.
But since a control input is always related to a certain level of vehicle deceleration, problems will arise during braking on a slope. Namely, while going down a slope, a given amount of braking force is required to only keep the vehicle running at a constant speed. However, when the manual brake control means are operated in an attempt to generate such a braking force, the brake fluid pressure will be controlled automatically so that a deceleration corresponding to the central input will be generated. Thus the vehicle will be decelerated instead of running at a constant speed.